The goal of this project is to develop a very inexpensive class of
volumetric displays that can present certain types of 3D content in a
compelling way. The types of content we wish to display are simple 3D objects,
extruded objects and 3D surfaces that appear dynamic when projected with
time-varying images. Our displays use a simple light engine and a dense cloud
of passive optical scatterers. The basic idea is to trade-off the (2D) spatial
resolution of the light engine to gain resolution in the third dimension. The
simplest way to achieve such a trade-off is to use a stack of planar grids of
scatterers where no two stacks overlap each other with respect to the
projection rays of the light engine. However, such a semi-regular 3D grid
suffers from poor visibility; as the viewer moves around the point cloud the
fraction of points visible to the viewer varies dramatically and is very small
for some of the viewing directions. Our key insight is to randomize the point
cloud in a manner that is consistent with the projection geometry. We have
verified that when a point cloud is randomized in a specific manner it produces
a remarkably stable visibility function.

We have explored several ways of creating dense clouds of passive
scatterers. We have chosen to use Laser Induced Damage (LID) technology which
can very efficiently, precisely, and at a very low cost embed the desired point
clouds in a solid block of glass. Each scatterer is a physical crack in the
glass that is created by focusing a laser beam at the point. When a point in
the cloud is lit by ambient light it is barely visible, but when it is lit by a
focused source it glows brightly. We have studied the radiometric and spectral
characteristics of LID scatterers and found that they have the properties
needed to display objects in color and with sufficient brightness to be viewed
within a 120 degree cone that is aligned with the projection direction. For
illuminating the scatterers, we have developed an orthographic light engine
that uses an off-the-shelf projector and inexpensive optics to create parallel
rays with a large footprint. Orthographic projection enables us to use point
clouds without resolution biases and makes the calibration of the display
relatively straightforward.

We have developed several versions of our volumetric display, each one
designed to meet the needs of a specific class of objects or a specific
application. We have implemented point clouds with 10,000 points for the
display of true 3D objects, 190,500 points for the display of extruded objects
with arbitrarily textured top surfaces, 180,500 points for the display of
purely extruded objects, 83,866 points for the extension of the game Pac-Man to
3D, and 127,223 points for the face model used to create a 3D avatar.

Video

Volumetric Displays:

This video describes the working principle of our volumetric displays as well
as several examples of objects shown using the displays. Examples of true 3D
objects and extruded objects are shown. In addition, the 3D Pac-man and 3D
Avatar are shown. (Apple Quicktime movie, with narration).